Compression Ratio Calculators

[Stealth-Z]

What is everyone using for a compression ratio calculator? I am in what seems to be a never ending RB30/26 build.

 

I have been using the calculator for the four stroke version on Wiseco's website.

http://www.wiseco.com/Calculators.aspx

 

Here are the specs on the Wiseco pistons I have.

3.046 inches (86.5mm)

1.260 inch (32mm) Comp Height

16cc dome

 

The RB26 head I have measures out at 64cc's. The head gasket being used is a Cometic MLS 87mm. Cometic rates the thickness at .051 inches (1.295mm). My block has been decked and the pistons sit at a zero deck.

 

When I plug these numbers,

Bore Diameter = 3.046 (86.5mm)

Engine Stroke = 3.346 (85mm)

Head Volume = 64cc

Gasket Bore Diameter = 3.425 (87mm)

Compressed Gasket Thickness = .051

Deck Clearance = 0

Piston Effective Dome Volume = 16cc

 

With these numbers I get a CR of 9.69:1

 

My plan is to shave nearly 11cc's off the piston dome to bring the dome volume to 5cc. According to the calculator this would give me CR of 8.490:1.

 

Now here is my issue. There is a huge section on RB30 conversions on Skylines Australia http://www.skylinesaustralia.com/forums/RB30-conversions-f184.html An individual who is very well known states pistons with a a 4.5cc dish sounds about right for 8.5 to 1 using a 62 cc combustion chamber. He says flat tops are good for 9.0 to 1.

 

His numbers do not seem right. Any one have an opinion? Are there different ways to measure and calculate that I am missing?

[Careless]

using the compression calculator at:

http://www.csgnetwork.com/compcalc.html

 

I get a ratio value of 9.96:1 with your stated specs.

 

dropping the pistons by 11cc gives you 8.49:1

 

changing the combustion chamber volume to 62cc and a 4.5cc dish on the pistons give you a ratio of 7.73:1. What he meant to say was a dome of 4.5 cc, which puts you at 8.66:1 with a combustion chamber vol of 62cc.

 

I try to use Dyno2003 software but there are two values in the software that have almost no bearing on any measurements i've seen as standard values used in these formulas, therefore I do not use that software at all because I think it's hard to apply "fake" values to see what I need to get to the desired comp ratio.

[Careless]

Also, if you were to zero the deck, your quench is 0.051. I think instead of going with cutting the pistons, you should use a 1 mm headgasket to get you a quench of .040 which is close to the 0.035 - 0.037 that people seem to find as a good value between too close and too far. it leaves good room for expansion of rods, that would close the piston to head clearance up by a tenth or so at a touch over operating temp.

 

64cc combustion chamber

5.5cc piston DOME

0 deck clearance

87mm x 1mm gasket

 

= 8.75

 

take 2cc of the combustion chamber

 

= 8.999:1

 

the extra 0.5mm on the gasket bore really does nothing to the compression ratio. However, I'm curious if that would become a hot spot.

[Stealth-Z]

changing the combustion chamber volume to 62cc and a 4.5cc dish on the pistons give you a ratio of 7.73:1. What he meant to say was a dome of 4.5 cc, which puts you at 8.66:1 with a combustion chamber vol of 62cc.

It is being claimed that using flat tops gives you 9:1. This is what does not make sense to me.

http://www.skylinesaustralia.com/forums/index.php?showtopic=212676&view=findpost&p=3938100

 

It is not the first time I have read this. The first set of Wiseco pistons had an equivelent of 5cc's dish.

http://forums.hybridz.org/showthread.php?t=119593&highlight=rb30+importance

 

No matter how I juggle the parameters, I still can not come up with the numbers being mentioned.

http://forums.hybridz.org/showpost.php?p=664964&postcount=18

 

I am trying to clarify this so when people go out and spend $$$ on pistons they get thier desired results. I am curious if there is another way to measure or determine CR.